A ball mill of $1.8 \mathrm{~m}$ diameter is loaded with steel balls each having a diameter of $6 \mathrm{~cm}$. The rotational speed of the ball is kept at $75 \%$ of the critical speed. The operational speed of the ball mill in rpm is______________
A ball mill of $1.8 \mathrm{~m}$ diameter is loaded with steel balls each having a diameter of $6 \mathrm{~cm}$. The rotational speed of the ball is kept at $75 \%$ of the critical speed. The operational speed of the ball mill in rpm is______________